On Sun, 4 Dec 2016 16:22:02 -0500
Scott Stobbe scott.j.stobbe@gmail.com wrote:

I guess you are refering to spread-spectrum techniques.
Such DC/DC converters exist, but are usually those with high power
ratings. IMHO it is also not worth the effort, as its main use is
to meet EMI emission requirements. The only application that comes
to my mind where spread-spectrum actually helps are high sensitiv
radio receivers where every spur is a nuisance. For most other
use, and time-nuts use in particular, it is much less useful.
The noise energy is not gone. It is still there, just spread over
a large bandwidth. In time measurement applications, noise is
integrated over time and frequency. Thus even if the noise is
spread over a large bandwidth, the energy will still contribute
to the uncertainty and degrade the ADEV. It will be just harder
to identify as the peak is now much smaller and wanders in frequency.

It is much better to the design such, that as little as possible
of the switching energy leaks out of the DC/DC converter and filter
out the rest.

Depending on the application, another possible application is to
sync up the DC/DC converter to the "main" clock source. This makes
the switching noise then coherent to the system, which either makes
it average out completely, or possible to filter it out in the digital
domain using a deep notch-filter in receiver applications.

		Attila Kinali

--
Malek's Law:
Any simple idea will be worded in the most complicated way.

On Sun, 4 Dec 2016 13:28:44 -0800
jimlux jimlux@earthlink.net wrote:

Wouldn't it be better to use a ferrite bead instead, for this application?
The much lower series capacitance and thus higher self-resonance frequency
should help damping the spurs.

Or am I missing something?

		Attila Kinali

--
Malek's Law:
Any simple idea will be worded in the most complicated way.

On Sun, 4 Dec 2016 16:41:13 -0500
John Ackermann N8UR jra@febo.com wrote:

This is exactly the PWM to PFM mode switch I described earlier.

The 850mA are probably high enough that the DC/DC converter works
in PWM mode, thus switching with a constant frequency, even if the
load changes. On the other hand 200mA of a 1A converter is low enough
that it's most likely in the PFM region, where every small change
(or noise) causes the switching frequency to change.

		Attila Kinali

--
Malek's Law:
Any simple idea will be worded in the most complicated way.

On 12/4/16 2:59 PM, Attila Kinali wrote:

sure, if you can get enough L.  The other thing is that SMD inductors
can be placed by machine, which isn't necessarily the case with ferrite
beads (depending on how you do them).

Well a sigma-delta modulator in loose terms is an error amplifier around a
quantizer, so you get 1/loopgain rejection of quantization noise (in other
words the noise is shaped out in frequency). Resulting in a noise spectrum
that converges in 1/N versus 1/sqrt(N) for flat Gaussian noise, versus
lobes and nulls for a single sinusoid.

I totally agree that the first step is to reduce the switching residual
that is generated, even half a bond wire at say 1 nH is 13 mOhms at 2 MHz,
combine that with a power converter running 30% ripple current of a 1A
output is 300mA ripple current resulting in 4 mV just on half a bondwire.
Equivalently a 10 uF MLCC should be able to hit 10 mOhms at 2 MHz before
hitting its SRF.

On Sun, Dec 4, 2016 at 5:58 PM, Attila Kinali attila@kinali.ch wrote:

On Sun, 4 Dec 2016 17:07:53 -0800
jimlux jimlux@earthlink.net wrote:

A BLM18 gets you into the order of 300Ω @ 100MHz, which is about
the equivalent of about 480nH. Using a lower current version gets
you into the 1kΩ @ 100MHz range (~1.6µH). Shouldn't that be enough
inductance?

Combined with some 10µF of capacitance and you already get ~15dB @ 1MHz
for a simple LC-lowpass. For additional filtering, add something like an
NFM18PS105 and you  get good filtering up to ~100MHz.

		Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson

Hi

The BLM18 gives you 300 ohms at zero current. It’s impedance at
zero current per the data sheet. If you look at other parts of similar
size, the impedance drops to nearly zero at the “rated current”. Great
if you have a signal lead and no current. Not so great if you have an
OCXO that is pulling 600 ma in a cold garage lab. A bead for close
to 1A use with 100 ohms at the 100 MHz would be much larger. At
the 10 MHz frequency of interest, anything close to 100 ohms  would
be even larger still.

Bob

Attila - I certainly do not differentiate between "ferrite bead" and "single turn toroidal choke".

I think the SMT inductor manufacturers think of them in the same bucket too.

what I think of as a "ferrite core for winding a multi turn inductor on", is invariably listed as a "ferrite bead" in the catalog these days.

Most of my switchers run in the very low MHz and I like to have at least several hundred ohms at that frequency before a bus or wiring run. This often means ten or more turns.

Tim N3QE

Sent from my iPhone

Not impressed with the PCB layout, could have been much better. Input cap is too far from regulator, connected to ground via longish (inductive) skinny trace. Input might benefit from simple PI filter to reduce input ripple current. Output caps should each have their own via, maybe even two each. Sense resistors should be near regulator chip, they aren't, so a longish high impedance path exists for noise susceptibility.

Bob L.

In my experience the term ferrite bead is used more loosely today. Most "beads" that I've designed with were 100% machine placeable surface mount parts from smallish 0402 chips on up to 10A rated honkers. Admittedly any tubular "bead on wire" is not machine placeable in a surface mount oriented factory, but these are becoming antiques.

Bob L.